This proposal has as its long term objective to delineate the molecular mechanisms by which are chemotactic peptide F-Met-Leu-Phe-OH (FMLP) interacts with its receptor substance. Studies will be carried out initially on receptor present in isolated plasma membranes from the promyelocytic leukemia HL-60 cell line. The factors (e.g. cations, G- proteins, cytoskeletal components) controlling expression of receptors will be evaluated in light of our recent observation that these receptors show a temperature and divalent cation dependent up regulation in plasma membranes. In addition we will extend our studies on the purification of the FMLP rceptor from the HL-60 cell line. This will utilize a new approach in that an affinity step utilizing a secondary capture technique will be employed. It will involve the use of a newly prepared high affinity analog, F-Met-Leu-Phe-Lys-(biotin)-OH (ED 50 lyosomal enzyme release - 4.8x10 -11 M) to link ligand bound receptor to avidin containing affinity supports. This approach should not only result in an additional level of selectivity for the affinity chromatography step but may also allow isolation of a ligand-free receptor protein. Finally, receptors will be studied to evaluate the thermodynamic characteristics of this ligand/receptor interaction. These experiments will ultimately be applied to isolated, reconstituted receptors to evaluate the "functional" state of the isolated protein.